1. Field of the Invention
The invention relates to detection and an imaging operation of a radiation such as X-ray, γ-ray, or the like and, more particularly, to a radiation detector, a radiation detector element, and a radiation imaging apparatus for detecting a γ-ray of high energy.
2. Description of the Related Art
When taking a medical X-ray as an example, as well as a radiation imaging apparatus of a film type, radiation imaging apparatuses such as imaging plate and flat panel detector (FPD) having both excellent resolving power and resolution have been developed. As a detector element, there has been used: a scintillation detector formed by combining a scintillator which reacts on a radiation and emits light and a photomultiplier or a photodiode which converts the light into charges; or a solid state device such as a semiconductor radiation detector which reacts on the radiation and directly collects generated charges or the like. For example, the FPD using a number of scintillation detectors is a large-area imaging apparatus which can image a transmission X-ray in a manner similar to a conventional X-ray imaging film. An X-ray signal detected in the detector element is read out from a detector element substrate of a large area comprising one or a plurality of sheets by using a TFT technique or the like. As a name “flat panel” shows that the detector element is very thin and the detector itself has a plate shape.
According to a gamma camera (radiation detector) for imaging a γ-ray emitted from a chemical-ray source dosed into a human body, since energy of the γ-ray which is used is higher than that of the X-ray, if the γ-ray is used as it is, sensitivity of the gamma camera deteriorates remarkably. That is, with a thickness of the detector element which is used in an X-ray imaging apparatus, a reaction probability of the γ-ray is low and the γ-ray passes through the detector element as it is. Therefore, to improve the sensitivity by raising the reaction probability in the detector element, the detector element needs to have a thickness of a certain extent in the incident direction of the γ-ray. That is, the detector element itself has directivity. Accordingly, unless the incident direction of the γ-ray is specified for such a detector having the directivity, space resolution cannot be obtained.
Generally, to specify the incident directivity of the γ-ray, a slit called a collimator or a thick porous metal plate is arranged in front of the detector. (Refer to “Medical Image•Radiation Apparatus Handbook”, Japan Industries Association of Radiological Systems, page 184)
FIG. 25 shows a construction of a conventional gamma camera disclosed in the above reference. At present, a gamma camera using an NaI scintillator is still a mainstream. The gamma camera of FIG. 25 also uses a similar scintillator 31. A radiation enters the scintillator 31 at an angle limited by a collimator 41e and scintillation light is emitted. The light reaches a photomultiplier 33 through a light guide 32 and becomes an electric signal. The electric signal is shaped by a measuring circuit 34 attached to a measuring circuit fixing board 35 and sent from an output connector 46e to an external data collecting system. A whole camera is enclosed in a light shielding casing 47e, thereby shielding external electromagnetic waves other than the radiation.
Generally, since the gamma camera using the scintillator 31 as shown in FIG. 25 has a structure in which the large photomultiplier 33 is arranged behind a crystal of large scintillator 31 of one sheet, its space resolution is no more than equal to about 10 mm. When the scintillator 31 is utilized, it detects the radiation via multi-level conversion from the radiation to the visible light and from the visible light to electrons, there is a problem such that energy resolution is low. Therefore, at present, a radiation detecting apparatus having a semiconductor radiation detector element for directly converting the radiation into the electric signal in place of the scintillator 31 has been developed. (“Radiation Detection and Measurement, the 3rd edition”, The Nikkan Kogyo Shimbun Ltd., page 903)
In a conventional gamma camera (semiconductor radiation detector) shown in FIG. 26A, a semiconductor device 77 has electrodes (an anode 78 and a cathode 79). The semiconductor device 77 has a construction in which the anodes 78 are arranged in a lattice form by the electrodes 78 and 79 (“Radiation Detection and Measurement, the 3rd edition”, The Nikkan Kogyo Shimbun Ltd., page 903). Reference numeral 41e denotes the collimator; 44′ a board for installing semiconductor device and an ASIC; 45c an ASIC (Application Specific Integrated Circuit) as an IC for a reading circuit; 46c an output connector to output a detection signal; and 47c a light shielding casing to shield the visible light and electromagnetic waves.
Also in the gamma camera, as same as in the FPD, realization of a large imaging area is indispensable. A number of detector elements are necessary in association with the realization of the large area. In the case of the scintillation detector, such a number of detector elements are separated as elements by the photomultiplier or photodiode disposed adjacently to one large device substrate. In the case of the semiconductor radiation detector, they are separated as elements by pattern wirings of the electrodes 78 and 79 as shown in FIG. 26B. To remove scattered components of the γ-ray, intensity information of the γ-ray is obtained by counting pulses. For this purpose, a preamplifier, a waveform shaping circuit, a peak detecting circuit, and the like are necessary for each element and an extremely large number of circuits are necessary in the case of a large area. Therefore, by using the ASIC 45c for those circuits, saving of space is realized.
In the conventional semiconductor radiation detector as shown in FIGS. 26A and 26B, however, even if the collimator 41e is used, the γ-ray scattered in the detector element 77 (scintillator 31) enters the adjacent cell and exercises an influence thereon. This kind of scattering radiation detection (refer to γ1′ in FIG. 14) and causes a deterioration in space resolution. To avoid an inconvenience caused by such a phenomenon, in the radiation detector, an incident position is identified by energy of the incident γ-ray (γ0). That is, since a reaction signal (ΔE00) near the energy of the γ-ray emitted from a γ-ray source 16d is discriminated and selectively detected, sensitivity deteriorates more. That is, the sensitivity of the radiation detector is extremely lowered by the inherent low sensitivity, the decrease in incident γ-ray due to the collimator 41e, and the discrimination of the energy. Although a hole diameter of the collimator 41e is increased and an incident dose is increased while sacrificing the space resolution in order to compensate the deterioration in sensitivity, the higher the energy of the γ-ray to be detected is, the thicker a wall of the collimator 41e has to be. Consequently, not only the space resolution deteriorates even more but a weight increases and maintenance efficiency of the radiation detector or the radiation imaging apparatus also deteriorates.
Since a number of radiation detector elements (pixels) are necessary for the large-area imaging, use of the ASIC and the element separation by electrode patterning of a signal lead-out portion are indispensable. However, they cause the following problems.                (1) An installing board of the detector and the ASIC are integratedly formed by bumping or the like and if one pixel is destroyed, the board has to be exchanged on a large unit basis. Since the detector element is very expensive, the exchange of the board on a large unit basis denotes that large costs are required.        (2) Also in view of the manufacturing of the camera, since the detector elements and the ASIC are installed onto one installing board, assembling steps of the camera are extremely complicated and, even if a defective element is found, it cannot be exchanged.        (3) Particularly in the radiation detector for imaging the high-energy γ-ray, a length of collimator is long, a total length of radiation imaging apparatus is very long, and it is very heavy and large in size. This causes an enlargement in size of the apparatus in terms of intensity of structure members which support a camera unit and results in an increase in costs, a deterioration in maintenance efficiency, and an increase in anxiety of the patient.        
In other words, it causes a deterioration in maintenance efficiency of the radiation detector and the radiation imaging apparatus.